This invention relates generally to the examination of a patient by means of radiation and, more particularly, to method and apparatus for tomographically examining an internal section of a patient by scanning a beam of X-radiation from a radiation source which orbits the patient.
1. Field of the Invention
A conventional radiograph is a two-dimensional shadow image of a three-dimensional object. The depth dimension is not apparent as all interior portions of the object appear to be in a single plane. As a consequence a conventional radiograph fails to provide detail as to three-dimensional spatial location of a condition. Under some conditions a conventional radiograph is difficult to interpret and it may not reveal a condition which exists.
The science of tomography developed which examined cross-sectional planes of a patient by sequentially bombarding the patient with X-rays from a plurality of directions. Conventional tomographic systems utilized a radiation sensitive recording film plate whose movement was coordinated with movement of a radiation source. The source-recording plate pair moved about a system axis passing through the patient and recorded a cross-sectional image of the patient in a plane which was transverse to the axis of the X-ray beam. The movement of the source-recording plate pair was such that elements in the selected cross-sectional plane of the patient were continuously scanned by the beam. This scanning technique resulted in movement on the film of images of the elements out of the selected plane, and these images were blurred with the result that air image of the selected plane was produced.
Conventional tomographic techniques resulted in loss of information, as elements in the other planes cast shadows in the selected cross-sectional plane of the patient. These shadows reduce the quality of the image recorded on the X-ray film as compared with transaxial techniques used in the present invention.
2. Prior Art
Variations were developed in forming the image which resulted from conventional tomographic scanning techniques. According to one proposal the recording plate was to be replaced with a radiation sensitive detector which orbited in aligned synchronism with the radiation source. More specifically, the source/detector pair was to be angularly rotated in a plane as the beam of radiation passed through the patient. The patient and source were to be periodically translated relatively in the plane of rotation and the rotation was then repeated. The angular rotation was to be about a system axis which passed through the patient, and the beam of radiation passed through the system axis. By passing the beam through the system axis as the source was to be rotated, a small central region within the patient could be isolated by cancelling the affects of all areas remote from the central region. Translation of the patient would allow an image of a section of the patient in the plane of rotation to be reconstructed as a video image displaying an integration of information from a series of small central areas. Tomography which produces an image in a plane which includes the X-ray beam axis is known as transverse section tomography.
Another proposal suggested the use of a plurality of radiation detectors disposed in a line in the direction of patient translation in an attempt to increase the effective allowable translation speed. Even with the multiple detectors, these proposals using an orbital scan motion coupled with a linear translation motion resulted in a system requiring long scanning times to provide images of limited size and quality.
A type of scanning system is described in Kuhl, et al., "Transmission Scanning, A Useful Adjunct to Conventional Emission Scanning For Accurately Keying Isotope Deposition To Radiographic Anatomy", Radiology, 1966, Vol. 87, pp. 278-284. The referenced "Emission" scanning system uses a detector for scanning a patient who has had radioisotope administered. The detector measures intensity values of the radiation as it is emitted. Transmission scanning differs from emission scanning in that it uses a radiation source to transmit a beam of radiation through the patient instead of radiation from an administered radioisotope. An emission scanning tomographic system is described in D. E. Kuhl and R. Q. Edwards, "Cylindrical and Section Radioisotope Scanning of the Liver and Brain", Radiology, Vol. 83, No. 5, pp. 926-936; 1964.
The science of reconstruction tomography using transverse section scanning has evolved to a system where a radiation source/detector pair scans a patient with a beam of radiation emitted as the source detector/pair are translated with the beam axis in a plane containing the section of the patient to be examined. The angular orientation of the beam is changed from one scan to another. The detected intensity of the beam is recorded for computing X-ray transmission or X-ray absorption characteristics through the scanned section. A plot of these characteristics provides a reliable image of the internal structure of the patient in the scanned plane. Transverse section scanning is also described in the above Kuhl, et al., references.
In one proposed reconstruction tomographic system using transverse section scanning, a linearly disposed array of source/detector pairs were to rectilinearly scan a patient along a path at a first angle with respect to an axis passing through the patient. Radiation intensity values were to be recorded during the rectilinear scan. After completing a rectilinear scan of the patient at the first angle the source/detector pair were to be angularly indexed. A second rectilinear scan was to be performed on the patient along a second path at a second angle with respect to the axis, and so forth.
After rectilinearly scanning along paths covering 180 degrees of angles with respect to the axis, the intensity data collected from radiation measurements were processed utilizing a method of successive approximations. A reconstructed image was generated representing the X-ray transmission of X-ray absorption coefficients lying in a section of the patient.
Apparatus for performing the rectilinear scans, was massive and required undue motive forces for linearly accelerating, decelerating and reversing direction of the array of the source-detector pair. This is better appreciated when understanding that, for a complete study, 180 scans were required. A complete study thus required 180 translational and 180 rotational accelerations and decelerations, as well as 180 direction changes during translation. In addition, the large number of accelerations, decelerations and direction changes resulted in a system requiring an undue amount of scanning time. A minimum scanning time is essential to minimize the time required for completing a study to thus protect the patient from excessive amounts of radiation and minimize the effects of changing conditions in the patient.
A prior proposal has suggested the transverse section scanning of a specimen by a source/detector pair which orbited about an orbiting system axis which passed through the patient. The system axis was slowly orbited to trace a circle of small diameter as the source-detector pair orbited about the system axis. The proposal failed to disclose how to implement a system which provided the required circular motion on the system axis. Apparatus for revolving the source/detector pair in such a proposal is subjected to extreme inertial forces due to the mass of the source/detector pair. Practicality required that the system axis be stationary if the source/detector pair is to be revolved with the precision required for exact image reconstruction. Furthermore, the proposal failed to disclose the relationships among the rotation about the system axis, the optimum angles of radiation measurement, the speed of orbit about the system axis, and the speed of rotation of the system axis in the circular motion.
The prior art has also suggested a transverse section, transmission scanning system which would orbit a source-detector pair about a system axis which passed through the patient. Concurrently with the orbiting the source-detector pair were to be rotated in the plane of the orbit about an orbiting source axis which passed through the source and which orbited about the system axis. The use of an array of detectors disposed symmetrically of the plane of orbit was also suggested. The proposal failed to disclose the relationships among the rotation of the system axis, the rotation of the source axis, and the orientations of the source/detectors at which measurements were to be taken if exact reconstruction was to be achieved. The reference also failed to disclose apparatus for implementing the suggested system. Futhermore, the suggestion did not recognize that data collected by this dual axial motion was not in the sequence required by the reconstruction algorithm disclosed with this suggestion. Without this recognition and a suggestion as to the solution of this problem, data collected by this scanning motion would not provide clinically acceptable results.